O'Brien began refereeing in 1984, combining it with working for the police for 17 years before turning professional in 1996. He quickly became a permanent fixture on the international scene and became New Zealand's most capped Test official in 2003.

He rose up the International Rugby Board ranks to the position of referee manager and was in charge of the sport's elite officials for seven years - including two Rugby World Cups.

He stepped down from his post in 2012 to take up a similar role within the Sevens game and was tasked with readying officials for the sport's return to the Olympics in Rio de Janeiro in 2016.